D. G. Kotsifaki, M. Kandyla, I. Zergioti, M. Makropoulou, E. Chatzitheodoridis, and A. A. Serafetinides
We present an optical nanotrapping setup that exhibits enhanced efficiency, based on localized plasmonic fields around sharp metallic features. The substrates consist of laser-structured silicon wafers with quasi-ordered microspikes on the surface, coated with a thin silver layer. The resulting optical traps show orders of magnitude enhancement of the trapping force and the effective quality factor.
DOI
Friday, July 6, 2012
Optical Tweezers-Assisted Cross-Correlation Analysis for a Non-intrusive Fluid Temperature Measurement in Microdomains
Chih-Ming Cheng, Ming-Chih Chang, Yu-Fen Chang, Wei-Ting Wang, Chien-Ting Hsu, Jing-Shin Tsai, Chia-Yeh Liu, Chien-Ming Wu, Keng-Liang Ou, and Tzu-Sen Yang
An image-based approach to predict the fluid temperature in microfluidic flow cell is presented. We apply Fourier-based cross-correlation processing to determine the lateral displacement of the optically trapped bead; therefore, both the mean square displacement (MSD) and the diffusion coefficient (D) can be obtained. On the other hand, applying the Stokes–Einstein equation, together with Faxen's law correction, the theoretical relation showed that D is proportional to (T/η), where Tand η are temperature and temperature-dependent fluid viscosity, respectively. Hence, the fluid temperature can be determined by MSD-based thermometry.
DOI
An image-based approach to predict the fluid temperature in microfluidic flow cell is presented. We apply Fourier-based cross-correlation processing to determine the lateral displacement of the optically trapped bead; therefore, both the mean square displacement (MSD) and the diffusion coefficient (D) can be obtained. On the other hand, applying the Stokes–Einstein equation, together with Faxen's law correction, the theoretical relation showed that D is proportional to (T/η), where Tand η are temperature and temperature-dependent fluid viscosity, respectively. Hence, the fluid temperature can be determined by MSD-based thermometry.
DOI
Thursday, July 5, 2012
Single-Molecule Measurements of the Binding between Small Molecules and DNA Aptamers
Philip M. Yangyuoru, Soma Dhakal, Zhongbo Yu,Deepak Koirala, Simon M. Mwongela, and Hanbin Mao
Aptamers that bind small molecules can serve as basic biosensing platforms. Evaluation of the binding constant between an aptamer and a small molecule helps to determine the effectiveness of the aptamer-based sensors. Binding constants are often measured by a series of experiments with varying ligand or aptamer concentrations. Such experiments are time-consuming, material nonprudent, and prone to low reproducibility. Here, we use laser tweezers to determine the dissociation constant for aptamer–ligand interactions at the single-molecule level from only one ligand concentration. Using an adenosine 5′-triphosphate disodium salt (ATP) binding aptamer as an example, we have observed that the mechanical stabilities of aptamers bound with ATP are higher than those without a ligand. Comparison of the change in free energy of unfolding (ΔGunfold) between these two aptamers yields a ΔG of 33 ± 4 kJ/mol for the binding. By applying a Hess-like cycle at room temperature, we obtained a dissociation constant (Kd) of 2.0 ± 0.2 μM, a value consistent with the Kdobtained from our equilibrated capillary electrophoresis (CE) (2.4 ± 0.4 μM) and close to that determined by affinity chromatography in the literature (6 ± 3 μM). We anticipate that our laser tweezers and CE methodologies may be used to more conveniently evaluate the binding between receptors and ligands and also serve as analytical tools for force-based biosensing.
DOI
Aptamers that bind small molecules can serve as basic biosensing platforms. Evaluation of the binding constant between an aptamer and a small molecule helps to determine the effectiveness of the aptamer-based sensors. Binding constants are often measured by a series of experiments with varying ligand or aptamer concentrations. Such experiments are time-consuming, material nonprudent, and prone to low reproducibility. Here, we use laser tweezers to determine the dissociation constant for aptamer–ligand interactions at the single-molecule level from only one ligand concentration. Using an adenosine 5′-triphosphate disodium salt (ATP) binding aptamer as an example, we have observed that the mechanical stabilities of aptamers bound with ATP are higher than those without a ligand. Comparison of the change in free energy of unfolding (ΔGunfold) between these two aptamers yields a ΔG of 33 ± 4 kJ/mol for the binding. By applying a Hess-like cycle at room temperature, we obtained a dissociation constant (Kd) of 2.0 ± 0.2 μM, a value consistent with the Kdobtained from our equilibrated capillary electrophoresis (CE) (2.4 ± 0.4 μM) and close to that determined by affinity chromatography in the literature (6 ± 3 μM). We anticipate that our laser tweezers and CE methodologies may be used to more conveniently evaluate the binding between receptors and ligands and also serve as analytical tools for force-based biosensing.
DOI
Scattering forces and electromagnetic momentum density in crossed circularly polarized standing waves
Manuel I. Marqués and Juan José Saénz
We analyze the forces on a small dipolar particle and the electromagnetic momentum density in a configuration consisting in two perpendicular circularly polarized stationary waves. The field distribution shows regions in which the electric and magnetic fields are parallel corresponding to a null Poynting vector. Although the average value of the momentum density, proportional to the Poynting vector, is zero in these regions, there are scattering forces acting on small particles due to light’s spin force. The total scattering force suggests a new definition of the average value of the momentum density for free propagating electromagnetic fields.
DOI
We analyze the forces on a small dipolar particle and the electromagnetic momentum density in a configuration consisting in two perpendicular circularly polarized stationary waves. The field distribution shows regions in which the electric and magnetic fields are parallel corresponding to a null Poynting vector. Although the average value of the momentum density, proportional to the Poynting vector, is zero in these regions, there are scattering forces acting on small particles due to light’s spin force. The total scattering force suggests a new definition of the average value of the momentum density for free propagating electromagnetic fields.
DOI
Observation of trapping and transporting air-borne absorbing particles with a single optical beam
Ze Zhang, Drake Cannan, Jingjiao Liu, Peng Zhang, Demetrios N. Christodoulides, and Zhigang Chen
We demonstrate optical trapping and manipulation of micron-sized absorbing air-borne particles with a single focused Gaussian beam. Transportation of trapped nonspherical particles from one beam to another is realized, and the underlying mechanism for the trapping is discussed by considering the combined action of several forces. By employing a specially-designed optical bottle beam, we observe stable trapping and optical transportation of light-absorbing particles from one container to another that is less susceptible to ambient perturbation.
DOI
We demonstrate optical trapping and manipulation of micron-sized absorbing air-borne particles with a single focused Gaussian beam. Transportation of trapped nonspherical particles from one beam to another is realized, and the underlying mechanism for the trapping is discussed by considering the combined action of several forces. By employing a specially-designed optical bottle beam, we observe stable trapping and optical transportation of light-absorbing particles from one container to another that is less susceptible to ambient perturbation.
DOI
Optical fiber nano-tip and 3D bottle beam as non-plasmonic optical tweezers
Samir K. Mondal, Sudipta Sarkar Pal, and Pawan Kapur
We report a simple fiber nano-tip as non-plasmonic optical tweezer, which can manipulate submicron particles in a non-contact manner. The efficiency of an optical tweezer can be enhanced by using non-diffracting type optical beams such as Bessel beam or self-imaged Bessel beam (3D bottle beam). The present work, for the first time, realizes a non-plasmonic optical tweezer based on a miniaturized axicon like single-mode optical fiber nano-tip. The tip generates non-diffracting type 3D bottle beam by virtue of its changing wedge angle. The nano-tip is prepared from a photosensitive single-mode optical fiber by employing a novel chemical etching technique. We experimentally demonstrate trapping of ~60 nm silver particle and ~160 nm silica particle using this nano-tip optical tweezer. The nano-tweezer also succeeds to pick up the particles from aqueous solution. The proposed nano-tweezer working at smaller laser powers opens new avenues for nanomanipulation and analysis of sub-microscale specimens in the biological and physical sciences.
DOI
We report a simple fiber nano-tip as non-plasmonic optical tweezer, which can manipulate submicron particles in a non-contact manner. The efficiency of an optical tweezer can be enhanced by using non-diffracting type optical beams such as Bessel beam or self-imaged Bessel beam (3D bottle beam). The present work, for the first time, realizes a non-plasmonic optical tweezer based on a miniaturized axicon like single-mode optical fiber nano-tip. The tip generates non-diffracting type 3D bottle beam by virtue of its changing wedge angle. The nano-tip is prepared from a photosensitive single-mode optical fiber by employing a novel chemical etching technique. We experimentally demonstrate trapping of ~60 nm silver particle and ~160 nm silica particle using this nano-tip optical tweezer. The nano-tweezer also succeeds to pick up the particles from aqueous solution. The proposed nano-tweezer working at smaller laser powers opens new avenues for nanomanipulation and analysis of sub-microscale specimens in the biological and physical sciences.
DOI
Wednesday, July 4, 2012
Trapping and deformation of microbubbles in a dual-beam fibre-optic trap
Susan E Skelton, Marios Sergides, Gianluca Memoli, Onofrio M Maragó and Philip H Jones
We present results of numerical calculations to evaluate the performance of a dual-beam fibre-optic trap for low refractive index particles such as ultrasound contrast agent microbubbles. Using a geometrical optics approach, we determine the range of parameters of microbubble size and beam dimensions over which the optical trap is stable and evaluate the trapping forces and spring constants. Additionally, we calculate the optically induced stress profile over the surface of the microbubble and evaluate the resulting deformation of the microbubble using elastic membrane theory. Our results suggest that such an experiment could be a useful tool for quantifying the mechanical properties (elastic modulus) of the shell material of an ultrasound contrast agent microbubble.
DOI
We present results of numerical calculations to evaluate the performance of a dual-beam fibre-optic trap for low refractive index particles such as ultrasound contrast agent microbubbles. Using a geometrical optics approach, we determine the range of parameters of microbubble size and beam dimensions over which the optical trap is stable and evaluate the trapping forces and spring constants. Additionally, we calculate the optically induced stress profile over the surface of the microbubble and evaluate the resulting deformation of the microbubble using elastic membrane theory. Our results suggest that such an experiment could be a useful tool for quantifying the mechanical properties (elastic modulus) of the shell material of an ultrasound contrast agent microbubble.
DOI
Intramolecular Folding in Human ILPR Fragment with Three C-Rich Repeats
Soma Dhakal, Javonne L. Lafontaine, Zhongbo Yu, Deepak
Enrichment of four tandem repeats of guanine (G) rich and cytosine (C) rich sequences in functionally important regions of human genome forebodes the biological implications of four-stranded DNA structures, such as G-quadruplex and i-motif, that can form in these sequences. However, there have been few reports on the intramolecular formation of non-B DNA structures in less than four tandem repeats of G or C rich sequences. Here, using mechanical unfolding at the single-molecule level, electrophoretic mobility shift assay (EMSA), circular dichroism (CD), and ultraviolet (UV) spectroscopy, we report an intramolecularly folded non-B DNA structure in three tandem cytosine rich repeats, 5'-TGTC4ACAC4TGTC4ACA (ILPR-I3), in the human insulin linked polymorphic region (ILPR). The thermal denaturation analyses of the sequences with systematic C to T mutations have suggested that the structure is linchpinned by a stack of hemiprotonated cytosine pairs between two terminal C4 tracts. Mechanical unfolding and Br2 footprinting experiments on a mixture of the ILPR-I3 and a 5′-C4TGT fragment have further indicated that the structure serves as a building block for intermolecular i-motif formation. The existence of such a conformation under acidic or neutral pH complies with the strand-by-strand folding pathway of ILPR i-motif structures.
DOI
Enrichment of four tandem repeats of guanine (G) rich and cytosine (C) rich sequences in functionally important regions of human genome forebodes the biological implications of four-stranded DNA structures, such as G-quadruplex and i-motif, that can form in these sequences. However, there have been few reports on the intramolecular formation of non-B DNA structures in less than four tandem repeats of G or C rich sequences. Here, using mechanical unfolding at the single-molecule level, electrophoretic mobility shift assay (EMSA), circular dichroism (CD), and ultraviolet (UV) spectroscopy, we report an intramolecularly folded non-B DNA structure in three tandem cytosine rich repeats, 5'-TGTC4ACAC4TGTC4ACA (ILPR-I3), in the human insulin linked polymorphic region (ILPR). The thermal denaturation analyses of the sequences with systematic C to T mutations have suggested that the structure is linchpinned by a stack of hemiprotonated cytosine pairs between two terminal C4 tracts. Mechanical unfolding and Br2 footprinting experiments on a mixture of the ILPR-I3 and a 5′-C4TGT fragment have further indicated that the structure serves as a building block for intermolecular i-motif formation. The existence of such a conformation under acidic or neutral pH complies with the strand-by-strand folding pathway of ILPR i-motif structures.
DOI
Exploring the complexity of aerosol particle properties and processes using single particle techniques
Ulrich K. Krieger , Claudia Marcolli and Jonathan P. Reid
The complex interplay of processes that govern the size, composition, phase and morphology of aerosol particles in the atmosphere is challenging to understand and model. Measurements on single aerosol particles (2 to 100 μm in diameter) held in electrodynamic, optical and acoustic traps or deposited on a surface can allow the individual processes to be studied in isolation under controlled laboratory conditions. In particular, measurements can now be made of particle size with unprecedented accuracy (sub-nanometre) and over a wide range of timescales (spanning from milliseconds to many days). The physical state of a particle can be unambiguously identified and its composition and phase can be resolved with a high degree of spatial resolution. In this review, we describe the advances made in our understanding of aerosol properties and processes from measurements made of phase behaviour, hygroscopic growth, morphology, vapour pressure and the kinetics of water transport for single particles. We also show that studies of the oxidative aging of single particles, although limited in number, can allow the interplay of these properties to be investigated. We conclude by considering the contributions that single particle measurements can continue to make to our understanding of the properties and processes occurring in atmospheric aerosol.
DOI
The complex interplay of processes that govern the size, composition, phase and morphology of aerosol particles in the atmosphere is challenging to understand and model. Measurements on single aerosol particles (2 to 100 μm in diameter) held in electrodynamic, optical and acoustic traps or deposited on a surface can allow the individual processes to be studied in isolation under controlled laboratory conditions. In particular, measurements can now be made of particle size with unprecedented accuracy (sub-nanometre) and over a wide range of timescales (spanning from milliseconds to many days). The physical state of a particle can be unambiguously identified and its composition and phase can be resolved with a high degree of spatial resolution. In this review, we describe the advances made in our understanding of aerosol properties and processes from measurements made of phase behaviour, hygroscopic growth, morphology, vapour pressure and the kinetics of water transport for single particles. We also show that studies of the oxidative aging of single particles, although limited in number, can allow the interplay of these properties to be investigated. We conclude by considering the contributions that single particle measurements can continue to make to our understanding of the properties and processes occurring in atmospheric aerosol.
DOI
Monday, July 2, 2012
Published Paper Statistics (first half of 2012)
Here is the results for the first half of 2012 for published papers on optical tweezers, micromanipulation and trapping.
The top Journals (more than 2% hits) are:
13,9% Optics Express
5,9% Optics Letters
5,1% Proceedings of the National Academy of Sciences
3,4% Applied Physics Letters
3,0% Nano Letters
3,0% Plos One
2,5% Physical Review Letters
2,1% Atti della Accademia Peloritana dei Pericolanti
Below is a tag cloud for the corresponding words found in the title and abstracts for the period:
Please submit your suggestions, comments, job posting and related conferences to the main Blog on Optical Tweezers website (http://opticaltweezers.blogspot.com), since it is then reposted to the social networks of Facebook and Google+.
The top Journals (more than 2% hits) are:
13,9% Optics Express
5,9% Optics Letters
5,1% Proceedings of the National Academy of Sciences
3,4% Applied Physics Letters
3,0% Nano Letters
3,0% Plos One
2,5% Physical Review Letters
2,1% Atti della Accademia Peloritana dei Pericolanti
Below is a tag cloud for the corresponding words found in the title and abstracts for the period:
Please submit your suggestions, comments, job posting and related conferences to the main Blog on Optical Tweezers website (http://opticaltweezers.blogspot.com), since it is then reposted to the social networks of Facebook and Google+.
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